The present invention relates to electrical power conversion apparatus, and more particularly, to DC-DC converters and, DC-AC inverters where two or more switching devices are switched in sequence, resulting in symmetrical switching, for example push-pull, full bridge and half bridge topologies.
Industry demands for increased density, that is more power transferred in a given volume, have led to the use of higher switching frequencies to minimize capacitor and magnetic core sizes. These higher frequencies require a greater attention to switching losses, which become a significant proportion of the total losses of the power conversion apparatus.
The sum of all the losses becomes even more important when an apparatus is required to be self-heatsinking. In a practical application, "power density" is best expressed as:
The amount of power delivered by a conversion device divided by the volume, defined by the extreme dimensions of the device expressed as a single geometric solid, and including a heatsink appropriate for a given airflow, MTBF (mean time before failure) and ambient temperature.
For high power density, it is necessary to increase the operating frequency of a converter in order to minimize the volume of magnetic components and capacitors. Whereas these volumes can thus be quite readily reduced, the volume of the heatsink is determined, for a given environment, by the power losses. Thus the installed power density is principally governed by the conversion efficiency. As the frequency is increased, switching losses of the switching and rectifying devices become significant, so a method of obtaining switching at zero voltage or zero current through the switching device at the time of switching is desirable.
With apparatus in which zero current switching is utilized, the dissipation caused by the product of the voltage across the switching device and the current through it, integrated over the period of the switch transition, is eliminated, but the dissipation caused by discharging the voltage on the parasitic capacitance of the switching device remains.
In zero voltage switching (ZVS), both the voltage current product integral and the discharge energy of the parasitic capacitance are eliminated. At higher switching frequencies, the parasitic capacitance energy is significant, therefore zero voltage switching is to be preferred for minimum losses.
Several techniques for obtaining ZVS have been proposed:
1. A Flyback quasi-resonant converter providing isolation has the disadvantage that high voltage stresses are presented to both the switching device and the output rectifier. Furthermore, ZVS is dependent on load current. At small load currents, ZVS does not occur.
2. A quasi-resonant forward converter has been proposed which achieves ZVS by using the magnetizing current of the transformer. The disadvantage is that high voltage, stresses are presented to the input switching device and the rectifier.
3. Charging the parasitic capacitance of the switching device with the magnetizing current and load current during the dead time (dead time being the period in which the switching device is turned off) has been used effectively on a full bridge converter and provides for pulse width modulated regulation A disadvantage is that an inductor in series with the transformer primary is required, and ZVS is dependent on load current.
4. A full bridge using the magnetizing current and the energy stored in the primary-secondary leakage inductance of the transformer has been proposed, but does not provide ZVS on all four switches.
5. A similar configuration can be used in a half bridge topology, with an inductor in series with the primary of the transformer. A disadvantage is, again, that ZVS depends on a certain minimum load. To achieve regulation, since the dead time must be maintained constant, operation at very high frequencies is necessary.
6. A further example is the high frequency power supply device disclosed in U.S. Pat. No. 4,553,199. All embodiments of this disclosure illustrated in FIGS. 1, 3, 4, 5, 6 and 7 of that patent show the use of a series inductive element and not the magnetizing inductance of the transformers and various methods are proposed for regulating the output.
These six examples illustrate the fundamental conclusion that, to achieve both ZVS and regulation within a converter, an inductance other than the magnetizing inductance of the transformer must be provided so as to give a ramp in the voltage of the parasitic capacitance in the dead time, during which the transformer is loaded. This conclusion gives rise to the necessary dependence on load current.